The causes of Achilles tendinitis are mainly overuse (especially in athletes), arthritis in the elderly, injury, bone spurs or infection.

There are two large muscles in the calf: the gastrocnemius and the soleus. These muscles are important for walking, and create the power needed to push off with the foot or go up on the toes. The large Achilles tendon connects these muscles to the heel.

The tendon can become inflamed due to overuse, arthritis, injury, or infection.

In young people, the most common cause is over use, particularly in athletes. Sports that involve extensive walking, running, and especially jumping, can put high levels of stress on the tendon which can lead to tendinitis over time.

In older people, arthritis is a more common cause of Achilles tendinitis.

Additionally, bone spurs or bone growths near the back of the heel can irritate the tendon, leading to tendinitis.